Golf ball

ABSTRACT

There is provided a golf ball providing a larger coefficient of restitution and producing a smaller hitting sound, characterized in that it provides a natural frequency under a fixed-free boundary condition (Fx) ranging from 1000 Hz to 1200 Hz both inclusive, and a natural frequency under a free boundary condition (Ff) ranging from 2200 Hz to 3700 Hz both inclusive, where Ff/Fx preferably ranges from 1.8 to 3.4 both inclusive.

This nonprovisional application is based on Japanese Patent Application No. 2004-081054 filed with the Japan Patent Office on Mar. 19, 2004, the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION

The present invention relates to a golf ball maintaining durability, achieving an increased flight distance, and producing a low hitting sound when hit.

As a technique for providing a golf ball with a larger coefficient of restitution and a longer flight distance, there has conventionally been provided a method of reducing a hysteresis loss of a golf ball material. Reducing the hysteresis loss thereof decreases an energy loss caused by deformation of the golf ball when hit with a golf club, increases energy transfer efficiency from the golf club to the golf ball, and thus provides an improved coefficient of restitution.

In order to increase a flight distance of the golf ball, a study has also been made on a matching between a golf ball and a golf club head. For example, there is provided a technique for setting closer to each other a frequency at which a mechanical impedance of the golf ball takes a primary minimum value and a frequency at which a mechanical impedance of the golf club takes a primary minimum value so as to provide a larger coefficient of restitution when hit (in U.S. Pat. No. 4,928,965, for example).

Based on the technique, a material of the golf club head suitable for conventional thread-wound golf balls and solid golf balls has shifted from persimmon to metal, and further to titanium alloy. With respect to a frequency at which a mechanical impedance of a club head made of each of the materials takes a primary minimum value, the titanium alloy head is usually designed to have Fx of approximately 1000 Hz under a fixed-free boundary condition, which is considerably smaller than that of the persimmon head of approximately 2000 Hz.

With respect to the golf ball, a study has conventionally been made only on its structure, blend, material and others in order to improve its coefficient of restitution and hitting sound for example, regardless of a property of the golf club. In contrast, with respect to the golf club, its coefficient of restitution, for example, has been improved based on a commonly-used golf ball's structure, blend and material, and a golf club having a metal head such as a titanium alloy head has recently been often used as the most suitable club. However, a study has not yet been made on a golf ball suitable for an iron golf club, and at the same time, having an improved coefficient of restitution and producing a low hitting sound.

SUMMARY OF THE INVENTION

The present invention provides a golf ball having an improved coefficient of restitution and producing a hitting sound with low frequency when hit with a golf club having a titanium alloy head, in particular, an iron club head.

The present invention provides a golf ball characterized in that it provides a natural frequency under a fixed-free boundary condition (Fx) ranging from 1000 Hz to 1200 Hz both inclusive and a natural frequency under a free boundary condition (Ff) ranging from 2200 Hz to 3700 Hz both inclusive. Herein a ratio (Ff/Fx) of the natural frequency under a free boundary condition (Ff) to the natural frequency under a fixed-free boundary condition (Fx) preferably ranges from 1.8 to 3.4 both inclusive.

The present invention in another aspect provides a golf ball characterized in that it provides a natural frequency under a fixed-free boundary condition (Fx) ranging from 1050 Hz to 1200 Hz both inclusive and a natural frequency under a free boundary condition (Ff) ranging from 2200 Hz to 3500 Hz both inclusive.

A ratio (Ff/Fx) of the natural frequency under a free boundary condition (Ff) to the natural frequency under a fixed-free boundary condition (Fx) more preferably ranges from 1.8 to 3.1 both inclusive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing a procedure for measuring a natural frequency under a fixed-free boundary condition (Fx).

FIG. 2 is a schematic view showing a vibrating method in measuring a natural frequency under a free boundary condition (Ff).

FIG. 3 is a schematic view showing a procedure for measuring the natural frequency under a free boundary condition (Ff).

FIG. 4 is a cross section of a golf ball according to the present invention.

FIG. 5 is a graph showing a relation between a coefficient of restitution and the natural frequency under a fixed-free condition (Fx).

FIG. 6 is a graph showing a relation between the coefficient of restitution and the natural frequency under a free boundary condition (Ff).

FIG. 7 is a graph showing a frequency transfer function (G(s)) and the natural frequency (Fx) in Example 1 under the fixed-free condition.

FIG. 8 is a graph showing a relation between a frequency transfer function (G′(s)) and the natural frequency (Ff) in Example 1 under the free condition.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a golf ball in which a natural frequency under a fixed-free boundary condition (Fx) is in a range of 1000 Hz to 1200 Hz both inclusive while a natural frequency under a free boundary condition (Ff) is in a range of 2200 Hz to 3700 Hz both inclusive.

FIG. 5 shows a relation between a coefficient of restitution and a value of the natural frequency under a fixed-free boundary condition (Fx). FIG. 5 shows that the coefficient of restitution increases as the value of Fx increases. Furthermore, the inventor has found that a frequency of the golf ball's hitting sound tends to increase as the value of the natural frequency under a fixed-free boundary condition (Fx) increases.

Furthermore, FIG. 6 shows a relation between the coefficient of restitution and a value of the natural frequency under a free boundary condition (Ff). FIG. 6 shows no correlation between the coefficient of restitution and the value of the natural frequency under a free boundary condition (Ff). However, the inventor has found that a frequency of the golf ball's hitting sound decreases as the value of Ff decreases. The present invention adjusts Fx contributing to the coefficient of restitution and Ff contributing to the hitting sound of the ball to fall within prescribed ranges, respectively, to keep balance between both of the properties.

In other words, if the natural frequency under a fixed-free boundary condition (Fx) is less than 1000 Hz, a coefficient of restitution would decrease. If it exceeds 1200 Hz, a frequency of the ball's hitting sound would increase. If the natural frequency under a free boundary condition (Ff) exceeds 3700 Hz, a frequency of the hitting sound thereof would increase. If it is less than 2200 Hz, a coefficient of restitution would decrease and a hit feeling would be too soft.

The natural frequency under a fixed-free boundary condition (Fx) is preferably in a range of 1050 Hz to 1200 Hz, more preferably of 1100 Hz to 1200 Hz while the natural frequency under a free boundary condition (Ff) is preferably in a range of 2200 Hz to 3500 Hz, in particular of 2200 Hz to 3300 Hz.

In order to further optimize the coefficient of restitution and the hitting sound of the ball, a ratio (Ff/Fx) of the natural frequency under a free boundary condition (Ff) to the natural frequency under a fixed-free boundary condition (Fx) preferably falls within a range of 1.8 to 3.4 both inclusive. If (Ff/Fx) is less than 1.8, a coefficient of restitution would tend to decrease. If (Ff/Fx) exceeds 3.4, a frequency of the ball's hitting sound would tend to increase. (Ff/Fx) preferably falls within a range of 1.8 to 3.1 both inclusive, particularly of 1.8 to 3.0 both inclusive.

Method for Measuring Fx

Based on FIG. 1 in the form of a schematic view, a method for measuring a natural frequency under a fixed-free boundary condition (Fx) will be described.

-   -   (A1) Grind a golf ball G to have a flat, circular portion of 10         mm in diameter, and fix the portion with instant adhesive to a         vibrator 17 on an attachment 17 a at a support 17 b;     -   (A2) Attach an acceleration pickup 19 under attachment 17 a of         vibrator 17;     -   (A3) Operate the vibrator to vibrate golf ball G to measure a         vibration rate V of the golf ball via a reflective tape 10         placed on an upper side of the golf ball, by means of a laser         radiation unit 14, a manipulator 12 and a laser Doppler         velocimeter 11. This utilizes a principle of a known laser         Doppler vibrometer. Note that the reflective tape is a Scotch         Light (trade name) reflection tape of SUMITOMO 3M Limited and         approximately 5 mm by 5 mm of the tape was stuck on the ball to         have a reflective surface thereof facing the laser radiation;     -   (A4) Amplify with a power amplifier 15 a voltage signal from         acceleration pickup 19, which is in turn taken into an FFT         analyzer 13. Meanwhile, take the measured rate V from laser         Doppler velocimeter 11 into FFT analyzer 13;     -   (A5) Calculate a frequency transfer function G(s) from an         acceleration A measured by FFT analyzer 13 and rate V, according         to the following expression:

G(s)=Fourier transform of output rate V/Fourier transform of input acceleration A; and

(A6) from frequency transfer function G(s), read a frequency of the highest peak value of peaks indicated for a range in frequency of 100 Hz to 2000 Hz. The read frequency is defined as a natural frequency under a fixed-free boundary condition (Fx).

Note that in FIG. 1, a vibrator amplifier 16 controls the vibrator 17 vibration amplitude and has a function amplifying a voltage signal output from FFT analyzer 13. A specification of the equipment used to measure the natural frequency under a fixed-free boundary condition (Fx) is shown in Table 1. TABLE 1 Equipment used to measure Fx measuring equipment manufacturer & type laser velocimeter DANTEC Co., Ltd. TRACKER MAIN UNIT TYPE55 N21 manipulator DANTEC Co., Ltd. 60X24 FFT analyzer HEWLETT PACKARD COMPANY DYNAMIC SIGNAL ANALYZER 3562A power amp PCB PIEZOTRONICS Inc. MODEL 482A18 vibrator amp SHINNIPPON SOKKI POWER AMPLIFER TYPE 360-B vibrator SHINNIPPON SOKKI 513-A acceleration PCB PIEZOTRONICS Inc. pickup MODEL 352B22

Method for Measuring Ff

A method for measuring a natural frequency under a free boundary condition (Ff) will now be described based on FIG. 2 showing a vibrating method and FIG. 3 schematically showing the measuring method.

-   -   (B1) Hang golf ball G with a thread 8 to be freely supported;     -   (B2) Attach an acceleration pickup 9 on one side of golf ball G;     -   (B3) Tap golf ball G lightly with an impulse hammer 2 on the         side opposite to acceleration pickup 9. As shown in FIG. 3, take         into the FFT signal analyzer a force F (an output signal of the         impulse hammer) and a response acceleration A (an output signal         of the acceleration pickup) generated at the time; and     -   (B4) Calculate by the FFT signal analyzer a transfer function         G′(s) below:     -   G′(s)=Fourier transform of an output acceleration A′/Fourier         transform of an input F′.

From the frequency transfer function, read a frequency of the highest peak value in a frequency range of 2000 Hz to 4600 Hz. The read frequency is defined as a natural frequency under a free boundary condition (Ff).

A specification of the measuring equipment used to measure the natural frequency under a free boundary condition (Ff) is shown in Table 2. TABLE 2 Equipment used to measure Ff measuring equipment type manufacturer FFT signal HP-3562A Yokogawa Hewlett Packard Ltd. analyzer (Japan) impulse hammer 086B03 PCB (PCB PIEZOTRONICS Inc.) (New York, U.S.) acceleration pickup 353B16 PCB (PCB PIEZOTRONICS Inc.) (New York, U.S.) power unit 480D06 PCB (both of impulse hammer (PCB PIEZOTRONICS Inc.) and acceleration pickup) (New York, U.S.)

Structure of Golf Ball

In the present invention, a golf ball can be a hollow golf ball, a golf ball having a thread-wound core, and furthermore a solid golf ball having a plurality of layers. FIG. 4 shows a cross section of a two-piece solid golf ball. A golf ball 20 here is made of a core 21 surrounded by a cover 22. By combining a structure and a type of material as appropriate, these golf balls can be formed such that a natural frequency under a fixed-free boundary condition (Fx) falls within a range of 1000 Hz to 1200 Hz, and that a natural frequency under a free boundary condition (Ff) falls within a range of 2200 Hz to 3700 Hz.

According to the present invention, Fx and Ff can be adjusted to fall within the ranges above by, for example, providing a golf ball with a multi-layered structure, and making the core's inner layer soft and the core's outer layer hard. Furthermore, Fx and Ff can also be adjusted by using for a cover a relatively hard material having a Shore D hardness of 55 to 67, preferably 58 to 66, particularly 60 to 65, and providing a relatively large thickness of 1.5 to 2.4 mm, preferably 1.6 to 2.3 mm, particularly 1.8 to 2.2 mm. A Shore D hardness of less than 55 would tend to decrease a coefficient of restitution while a Shore D hardness exceeding 67 would increase a frequency of the ball's hitting sound. A cover thickness of less than 1.5 mm would provide a golf ball with poor durability while a cover thickness exceeding 2.4 mm would increase a frequency of the ball's hitting sound.

Furthermore, the golf ball's amount of deformation by compression is adjusted to fall within a range of 2.6 to 3.7 mm, preferably 2.7 to 3.6 mm, particularly 2.8 to 3.5 mm. An amount of deformation by compression thereof of less than 2.6 mm would increase a frequency of the ball's hitting sound while an amount thereof exceeding 3.7 mm would decrease a coefficient of restitution.

Furthermore, the core's amount of deformation by compression is adjusted to fall within a range of 3.0 to 4.3 mm, preferably 3.2 to 4.2 mm, particularly 3.4 to 4.2 mm. An amount of deformation by compression thereof of less than 3.0 mm would produce a high hitting sound of the ball while an amount thereof exceeding 4.3 mm would decrease a coefficient of restitution.

Furthermore, Fx and Ff above can also be adjusted by producing a golf ball having a core with a hollow structure. Note that Fx and Ff can be adjusted by using one of the abovementioned techniques, or by combining the techniques as appropriate.

In the present invention, the core is not limited to a solid core and it can alternatively be a thread-wound core, for example a liquid or solid center with rubber thread wound therearound. A solid core or a solid center used for the thread-wound core according to the present invention is formed of a rubber composition crosslinked.

Composition of Solid Core

In the present invention, the rubber composition of the core contains a rubber component containing a base material suitably of butadiene rubber having a cis-1,4-strucuture, although the above butadiene rubber may be replaced for example by 40% by weight of natural rubber, styrene butadiene rubber, isoprene rubber, chloroprene rubber, butyl rubber, ethylene propylene rubber, ethylene propylene diene rubber, acrylonitrile rubber or the like blended for 100 parts by mass of the rubber component.

The rubber composition is crosslinked or co-cured with an agent, for example acrylic acid, methacrylic acid or any other similar α,β-ethylenic unsaturated carboxylic acid and zinc oxide or any other similar metal oxide that react during the preparation of the rubber composition and provide a metallic salt of α,β-ethylenic unsaturated carboxylic acid, or zinc acrylate, zinc methacrylate or any other similar metallic salt of α,β-ethylenic unsaturated carboxylic acid, polyfunctional polymer, N,N′-phenylbismaleimide, sulfur or any other similar substance typically used as a cross linker. In particular, zinc salt of α,β-ethylenic unsaturated carboxylic acid is preferable.

If a metallic salt of α,β-ethylenic unsaturated carboxylic acid is used as a cross linker or a co-curing agent, 10 to 40 parts by mass thereof is blended for 100 parts by mass of the rubber component. If α,β-ethylenic unsaturated carboxylic acid and a metal oxide react during the preparation of the rubber composition, 15 to 30 parts by mass of α,β-ethylenic unsaturated carboxylic acid and 15 to 35 parts by mass of zinc oxide or any other similar metal oxide for 100 parts by mass of the α,β-ethylenic unsaturated carboxylic acid can be blended together.

For the rubber composition, a filler can be used, such as one or more of barium sulfate, calcium carbonate, clay, zinc oxide or any other similar, inorganic powder. 5 to 50 parts by mass of such filler is preferably blended for 100 parts by mass of the rubber component.

Furthermore, for example to enhance workability and adjust hardness, a softener, liquid rubber, or the like may be blended as appropriate, and anti-oxidant may also be blended as appropriate.

Furthermore, cross-linking is started by a cross-link initiator such as dicumylperoxide, 1,1-bis (t-butylperoxy) 3,3,5-trymethylcyclohexane, or any other similar, organic peroxide. 0.1 to 5 parts by mass, preferably 0.3 to 3 parts by mass of such a crosslink initiator is blended for 100 parts by mass of the rubber composition.

A part of the solid core can be formed by a material of thermoplastic elastomer or a thermoplastic resin, or alternatively, a combination thereof.

Production of Solid Core

A solid core according to the present invention can be produced by a known method. For producing a solid core having a plurality of layers, for example, a blended material is mixed together by means of a roll, a kneader, a Bunbury mixer or the like to prepare a rubber composition, which is introduced into a mold having top and bottom portions each having a semispherical cavity, vulcanized under pressure at 145° C. to 200° C. for example, preferably 150° C. to 175° C., for 10 to 40 minutes to produce a core center.

A chaplet having an outer diameter equal to the core center is then arranged in a mold with a larger, inner, semispherical cavity. A rubber composition to be used for a second layer of the core is introduced therein and heated at a prescribed temperature for a prescribed time to produce a semi-crosslinked half shell. The mold is opened and the chaplet is removed to obtain the half shell for the second layer. The core center having upper and lower portions each covered with the half shell for the second layer is placed in a mold and thus further vulcanized to integrate the core center and the second layer of the core. This series of operations is repeated for a third and further layers of the core, and a solid core formed of a plurality of layers can be produced.

A surface of the outermost layer of the solid core obtained by the method above may have an adhesive applied thereon or be roughened to improve adhesion to the cover.

A diameter of the solid core is designed within a range of 36.8 to 41.4 mm, preferably 37.8 to 40.8 mm. A diameter of less than 36.8 mm would cause a thick cover layer and decrease a coefficient of restitution while a diameter exceeding 41.4 mm would cause a thin cover layer, making a molding thereof difficult.

Cover

In the present invention, a cover is preferably provided with a relatively large thickness. The cover may be formed of a single layer or a plurality of layers. In the case of a plurality of layers, an outer cover and an inner cover preferably use materials different in hardness.

The cover according to the present invention contains thermoplastic resin, e.g., ionomer resin, polyethylene, polypropylene, polystyrene, ABS resin, methacryl resin, polyethyleneterephthalate, ACS resin, polyamide or any other similar, general-purpose resin, although it preferably contains ionomer resin.

The ionomer resin is a copolymer of α-olefin and α, β-unsaturated carboxylic acid of a carbon number of 3 to 8 with a carboxyl group thereof at least partially neutralized with a metallic ion to provide a binary copolymer. It may alternatively be a terpolymer of α-olefin, α, β-unsaturated carboxylic acid of a carbon number of 3 to 8 and α, β-unsaturated carboxylate of a carbon number of 2 to 22 with a carboxyl group thereof at least partially neutralized with a metallic ion.

If the ionomer resin has a composition containing a base polymer of α-olefin and α, β-unsaturated carboxylic acid of carbon number of 3 to 8 to provide a binary copolymer, it preferably contains 80 to 90% by weight of α-olefin and 10 to 20% by weight of α, β-unsaturated carboxylic acid. If the base polymer is the terpolymer of α-olefin, α, β-unsaturated carboxylic acid of a carbon number of 3 to 8 and α, β-unsaturated carboxylate of a carbon number of 2 to 22, it preferably contains 70 to 85% by weight of α-olefin, 5 to 30% by weight of α, β-unsaturated carboxylic acid and 5 to 25% by weight of α, β-unsaturated carboxylate. These ionomer resins preferably have a melt index (MI) of 0.1 to 20. Carboxylic acid or carboxylate contained in the above range can increase a coefficient of restitution.

The α-olefin is for example ethylene, propylene, 1-butene, 1-pentene or the like and ethylene is particularly preferable. α, β-unsaturated carboxylic acid of a carbon number of 3 to 8 is for example acrylic acid, methacrylic acid, fumaric acid, maleic acid or crotonic acid and acrylic acid or methacrylic acid is particularly preferable. Unsaturated carboxylate is for example methyl ester, ethyl ester, propyl ester, n-butyl ester or isobutyl ester for example of acrylic acid, methacrylic acid, fumaric acid or maleic acid. Acrylic ester or methacrylic ester is particularly preferable.

The copolymer of α-olefin and α, β-unsaturated carboxylic acid or the terpolymer of α-olefin, α, β-unsaturated carboxylic acid and α, β-unsaturated carboxylate has a carboxyl group at least partially neutralized with a metallic ion such as sodium ion, lithium ion, zinc ion, magnesium ion or potassium ion.

The above ionomer resin specifically exemplified under trade name includes an ionomer resin of a binary copolymer commercially available from DuPont-Mitsui Polychemical Co., Ltd. such as Hi-milan 1555 (Na), Hi-milan 1557 (Zn), Hi-milan 1605 (Na), Hi-milan 1706 (Zn), Hi-milan 1707 (Na), Hi-milan AM 7318 (Na), Hi-milan AM 7315 (Zn), Hi-milan AM 7317 (Zn), Hi-milan AM 7311 (Mg), Hi-milan MK 7320 (K) and the like, and an ionomer resin of a terpolymer such as Hi-milan 1856 (Na), Hi-milan 1855 (Zn), Hi-milan AM 7316 (Zn) and the like.

Furthermore, DuPont Co., Ltd. commercially provides ionomer resin under the trade names of Surlyn 8140 (Na), Surlyn 8320 (Na), Surlyn 8940 (Na), Surlyn 8945 (Na), Surlyn 9120 (Zn), Surlyn 9910 (Zn), Surlyn 9945 (Zn), Surlyn 7930 (Li) and Surlyn 7940 (Li), and the terpolymer-type ionomer resin such as Surlyn AD 8265 (Na), Surlyn AD 8269 (Na) and the like.

Furthermore, Exxon Chemical Japan Ltd. commercially provides the ionomer resin under the trade names of lotek 7010 (Zn), Iotek 8000 (Na), and the like. Note that the above trade names of ionomer resin are followed by parenthesized symbols Na, Zn, K, Li, Mg and the like, which indicate metal types of these neutralizer metallic ions. Furthermore in the present invention the ionomer resin used for the cover composition may be a mixture of two or more of the above exemplified ionomer resins or a mixture of one or more of the above exemplified, monovalent metallic ion neutralized, ionomer resins and one or more of the above exemplified, divalent metallic ion neutralized, ionomer resins.

In the present invention, the cover can be formed of the thermoplastic elastomer, for example, styrene-type thermoplastic elastomer, urethane-type thermoplastic elastomer, ester-type thermoplastic elastomer, olefin-type thermoplastic elastomer, amide-type thermoplastic elastomer and the like.

The styrene-type thermoplastic elastomer is a block copolymer having a molecule with soft and hard segments therein. The soft segment is a unit for example of a butadiene block, an isoprene block or the like obtained from a conjugated diene compound.

The conjugated diene compound can be one or more selected for example from butadiene, isoprene, 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene-and the like, preferably butadiene, isoprene and a combination thereof. The hard segment is constituted by a unit for example of a styrene block obtained from a compound with one or more selected for example from styrene and a derivative thereof, e.g., α-methylstyrene, vinyl toluene, p-third butylstyrene, 1,1-diphenylethylene and the like. In particular, styrene block unit is suitable.

More specifically the styrene-type thermoplastic elastomer for example includes a styrene-isoprene-butadiene-styrene block copolymer (an SIBS structure), a styrene-butadiene-styrene block copolymer (an SBS structure), the SBS structure having butadiene with a double bond hydrogenated, or a styrene-ethylene-butylene-styrene block copolymer (an SEBS structure), a styrene-isoprene-styrene block copolymer (an SIS structure), the SIS structure having isoprene with a double bond hydrogenated, or a styrene-ethylene-propylene-styrene block copolymer (an SEPS structure), a styrene-ethylene-ethylene-propylene-styrene copolymer (an SEEPS structure), and these copolymers modified.

Note that the above SIBS, SBS, SEBS, SIS, SEPS and SEEPS structures contain 10 to 50% by weight, in particular 15 to 45% by weight of styrene (or a derivative thereof). If the copolymers contain less than 10% by weight of styrene the cover would be too soft and cut-resistance would tend to reduce and values of Fx and Ff would be difficult to adjust.

In the present invention the SIBS, SBS, SEBS, SIS, SEPS and SEEPS structure copolymers may partially be a modified product provided via a functional group selected from the group of an epoxy group, a hydroxy group, an acid anhydride, and a carboxyl group.

The present cover composition can contain the aforementioned thermoplastic resin and thermoplastic elastomer as a polymer component, independently or mixed together. If they are mixed, not more than 50% by weight of the thermoplastic elastomer is preferably mixed for 100 parts by mass of the polymer component to obtain a high elastic modulus.

Mixing ionomer resin or any other similar thermoplastic resin and thermoplastic elastomer together can provide the cover composition with an appropriate level of elasticity and achieve a satisfactory hit feeling. Furthermore in the present invention a short organic fiber, such as, nylon fiber, acrylic fiber, polyester fiber, aramid fiber or the like can be blended to increase the cover's elastic modulus.

EXAMPLES Examples 1 to 5 and Comparative Examples 1 to 3

(1) Production of Solid Core

For each of the cores in Examples other than Example 4, and for each of those in Comparative Examples 1 to 3, a material with the composition of blending shown in Table 3 was mixed together by means of a kneader to prepare a rubber composition for the solid core. The rubber composition was introduced into a core mold having top and bottom portions each having a semispherical cavity, and vulcanized under the condition shown in Table 3 to produce a core.

For the core in Example 4, a material with the composition of blending shown in Table 3 was mixed together by means of a kneader to prepare a rubber composition for the solid core. The rubber composition was introduced into a mold with a semispherical, convex chaplet and press-vulcanized at 170° C. for 10 minutes to produce a half shell. Two such half shells were attached to each other with an adhesive to obtain a spherical inner layer core having a thickness of 5 mm. The inner layer core was covered with two of semi-vulcanized half shells produced in advance with a rubber composition having the same blending, then introduced into a core-forming mold, and vulcanized and thus molded under the condition shown in Table 3 to obtain a core of 38.4 mm in diameter with a hollow having a diameter of 15 mm. TABLE 3 example comparative example 1 2 3 4 5 1 2 3 core blend BR-18^((note 1)) 100 100 100 100 100 100 100 100 zinc acrylate 31 24 28 20 31 22 22 33 zinc oxide^((note 4)) weight adjustment diphenyldisulfide^((note 2)) 0.5 0.5 0.5 — 0.5 0.5 0.5 0.5 anti-oxidant — — — 0.5 — — — — dicumylperoxide^((note 3)) 0.7 0.7 0.7 1.2 0.7 0.7 0.7 0.7 condition of temperature (° C.) 170 170 170 157 170 170 170 170 vulcanization time (minute) 15 15 15 20 15 15 15 15 property diameter of core (mm) 39.2 38.4 39.2 38.4 38.8 39.4 37.6 39.2 core's amount of deformation 3.4 4.1 3.7 4.2 3.4 4.4 4.2 3.2 by compression (mm) cover blend Hi-milan 1605^((note 5)) 50 50 50 50 Hi-milan 1706^((note 6)) 50 50 50 50 Surlyn 8140^((note 7)) 50 50 50 Surlyn 9120^((note 8)) 50 50 50 Hi-milan 1856^((note 9)) 100 titanium dioxide 4 4 4 4 4 4 4 4 property cover hardness 62 62 65 65 62 62 65 52 (Shore D hardness) cover thickness (mm) 1.8 2.2 1.8 2.2 2 1.7 2.6 1.8 ball's amount of deformation by 2.8 3.4 2.7 3.4 2.7 3.8 2.5 2.8 compression (98-1274N) golf ball Fx 1010 1010 1185 1190 1010 1005 1220 810 property Ff 3610 2220 3620 2210 2800 2030 3650 3850 Ff/Fx 3.6 2.2 3.1 1.9 2.8 2.0 3.0 4.8 coefficient of restitution 0.783 0.781 0.791 0.792 0.785 0.77 0.79 0.75 hitting sound (when hit with 4 5 4 5 4.5 5 2 3 iron) structure of ball 2- 2- 2- hollow 2- 2- 2- 2- piece piece piece piece piece piece piece durability (index) 115 110 110 100 115 50 60 130 ^((note 1))BR-18: available from JSR, high cis-polybutadiene (cis-1,4-bond content = 96%) ^((note 2))diphenyldisulfide: available from SUIMITOMO SEIKA CHEMICALS CO., LTD. ^((note 3))dicumylperoxide: available from NOF Corporation ^((note 4))zinc oxide: core weight was adjusted with zinc oxide to obtain a golf of 42.8 mm in outer diameter and 45.4 g in weight. ^((note 5))Hi-milan 1605: available from DuPont-Mitsui Polychemical Co., Ltd., ionomer neutralized with Na ^((note 6))Hi-milan 1706: available from DuPont-Mitsui Polychemical Co., Ltd., ionomer neutralized with Zn ^((note 7))Surlyn 8140: available from DuPont Co., Ltd., USA, ionomer neutralized with Na ^((note 8))Surlyn 9120: available from DuPont Co., Ltd., USA, ionomer neutralized with Zn ^((note 9))Hi-milan 1856: DuPont-Mitsui Polychemical Co., Ltd., ionomer neutralized with Na

(2) Preparation of Composition for Cover

A composition for a cover, as presented in Table 3, was mixed by means of a two-shaft kneading type extruder and it was extruded by the extruder at a cylinder temperature of 180° C. applied. The composition was extruded under the condition of a screw having a diameter of 45 mm, a rotation rate of 200 rpm and an L/D of 35.

The composition for the cover was used to injection-mold a semispherical, half shell and two such half shells were used to cover the above obtained core. It was then placed in a mold and at 150° C. pressed, thermally compressed and molded. After it was cooled the golf ball was removed from the mold and then had a surface painted and a golf ball of 42. 8 mm in diameter and 45.4 g in weight was thus produced. The golf ball thus produced had its physical properties and golf ball performance estimated, as described below.

(1) Measurement of Fx and Ff

For the golf ball above, a natural frequency under a fixed-free boundary condition (Fx) was measured according to the procedure shown in FIG. 1. A natural frequency under a free boundary condition (Ff) was also measured according to the procedure shown in FIG. 2. Values of Fx and Ff for respective Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 3.

FIG. 7 shows a chart of frequency transfer function (G(s)) and Fx of Example 1 as measurements, where Fx corresponds to 1010 Hz. Furthermore, FIG. 8 shows a chart of frequency transfer function (G′(s)) and Ff of Example 1 as measurements, where Ff corresponds to 3610 Hz.

(2) Coefficient of Restitution

A resilience gun was activated at an initial velocity of 40 m/s to a golf ball at rest to have the gun's cylinder collided against the golf ball. The gun's velocities immediately before and after collision V1 and V1′, respectively, and the ball's velocity after collision V2′ were measured and used to calculate a coefficient of restitution e by utilizing a law of conservation of momentum, according to the following expression: e=(V 2 ′−V 1′)/V 1

(3) Hitting Sound (Index)

A No. 5 iron club was attached to a swing robot to hit a golf ball at a head speed of 34 m/sec to measure a hitting sound thereof. The hitting sound was collected with a microphone, which was spaced 1.8 m apart from the golf ball to be hit, and placed 1600 mm above the ground to face the golf ball. The sound collected from the microphone was recorded on a recording medium. A frequency of the hitting sound was evaluated on a scale of 1 to 5, where 1 indicates the highest frequency thereof while 5 indicates the lowest frequency thereof

(4) Amount of Deformation by Compression

An amount of deformation (mm) was measured under an initial load of 98N to a final load of 1275N applied to a core or a golf ball.

(5) Shore D Hardness

Hardness was measured by means of a Shore D, spring type hardness tester defined in ASTM-D2240.

(6) Durability (Index)

A No. 5 iron club was attached to a swing robot of True Temper Sports to hit each golf ball at a head speed set to 34 m/sec to cause it to collide against a collision plate for evaluation. To establish criteria for evaluation, the number of impinges by which a golf ball was broken was measured and indicated as an index relative to that of Example 4, which was set to 100. Larger indexes indicate that the golf ball has superior durability.

As shown in Table 3, each of Examples 1 to 5 adjusts a natural frequency under a fixed-free boundary condition (Fx) and a natural frequency under a free boundary condition (Ff) to fall within a range of 1000 to 1200 Hz and a range of 2200 to 3700 Hz, respectively. Therefore, there can be obtained a golf ball maintaining durability, producing a hitting sound with low frequency, and providing an improved coefficient of restitution.

Comparative Example 1 having small Ff produces a hitting sound with low frequency, although it exhibits poor durability and decreased coefficient of restitution. Comparative Example 2 having large Fx provides a larger coefficient of restitution, although it produces a hitting sound with high frequency and exhibits poor durability.

Comparative Example 3 having small Fx provides a smaller coefficient of restitution, produces a hitting sound at a normal frequency level, and exhibits favorable durability.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the terms of the appended claims, and equivalence of the scope of the claims and all modifications that fall within the scope thereof being intended to be embraced.

As described above, the present invention can provide a golf ball superior in hit feeling and coefficient of restitution by adjusting a natural frequency under a fixed-free boundary condition (Fx) and a natural frequency under a free boundary condition (Ff) to fall within prescribed ranges, respectively. 

1. A golf ball providing a natural frequency under a fixed-free boundary condition (Fx) ranging from 1000 Hz to 1200 Hz both inclusive and a natural frequency under a free boundary condition (Ff) ranging from 2200 Hz to 3700 Hz both inclusive.
 2. The golf ball according to claim 1, wherein a ratio (Ff/Fx) of the natural frequency under a free boundary condition (Ff) to the natural frequency under a fixed-free boundary condition (Fx) ranges from 1.8 to 3.4 both inclusive.
 3. A golf ball providing a natural frequency under a fixed-free boundary condition (Fx) ranging from 1050 Hz to 1200 Hz both inclusive and a natural frequency under a free boundary condition (Ff) ranging from 2200 Hz to 3500 Hz both inclusive.
 4. The golf ball according to claim 1, wherein a ratio (Ff/Fx) of the natural frequency under a free boundary condition (Ff) to the natural frequency under a fixed-free boundary condition (Fx) ranges from 1.8 to 3.1 both inclusive. 